1. Field of the Invention
The instant invention relates to high strength adhesives, particularly those having low coefficients of moisture expansion, such as those based on monomers or oligomers of cyanate esters. The invention furthermore relates to composite adhesives, wherein the matrix of the composite comprises the adhesive, and the filler or additive comprises one or more strong or elastically rigid materials.
2. Discussion of Related Art
For certain applications, the preferred means of fastening or joining one structure to another is by gluing or adhesive bonding. Adhesives used to bond components used in applications requiring precise motion control should be stable with respect to mechanical loads, temperature changes and changes in humidity. For mechanical stability, an adhesive should be stiff and strong. For thermal stability, an adhesive should have a low coefficient of thermal expansion (CTE) and a high thermal conductivity. For moisture stability, an adhesive should absorb as little water as possible and have a low coefficient of moisture expansion (CME).
To elaborate further on this moisture stability issue, consider, for example, a lithography machine for semiconductor fabrication featuring an optical projection mirror mounted on a cantilever beam, the end of which is adhesively bonded to a supporting structure. If one side of the adhesive joint is kept in a moisture-free environment but the other is exposed to the ambient atmosphere, it is conceivable that the side of the joint exposed to the atmosphere will absorb some water vapor and expand, whereas the side of the joint kept moisture-free will remain in its original size. This differential expansion will tend to bend the cantilever beam, which in turn will change the path of light reflected from the mirror mounted on the cantilever beam. An optics system whose light path is affected by the humidity of the ambient atmosphere is not a robust design.
The cyanate esters are a class of resins that in general have favorable moisture stability characteristics. They also possess high tensile and lap shear strengths. While epoxies generally possess adequate strength for these applications, they have higher CME's than do the cyanate esters, and also absorb greater quantities of moisture than the equivalent amount of cyanate ester.
Other properties of the adhesive resin that may be important are its coefficient of thermal expansion (CTE) and its thermal conductivity, among others. The CTE of the adhesive is important for much the same reason as described above for CME. Even if the entire adhesive joint were to heat up uniformly, the joint would expand. Typically, the CTE of adhesives is greater, often much greater, than the materials to which they are bonded. Thus, the adhesive would be expanding much more than the bonded materials for the same rise in temperature. Such differential expansions are rarely beneficial, as they often create internal stresses that can potentially lead to strains or distortions.
Similarly, the thermal conductivity of polymer adhesives typically is low, at least compared to that of the materials that it bonds, such as metals, ceramics and composites. Where thermal insulating properties are required, the neat resin (e.g., with no added filler) may perform entirely satisfactorily. Semiconductor fabrication operations such as lithography, however, generate heat that needs to be dissipated, thus requiring high thermal conductivity materials. Here the low thermal conductivity of the adhesive is a disadvantage, and it would be desirable to have an adhesive whose thermal conductivity is closer to that of the substrates bonded by the adhesive.
Japanese Laid-open Patent Application No. JP 2-175,148 to Kouichi et al. discloses an adhesive for bonding a skin layer of thermoplastic resin to a core material. The adhesive may be a curing type system such as epoxy, urethane or acrylic, or a hot melt system such as ethylene-vinyl acetate, polyester or polyamide. A filler is added to the adhesive to reduce the thermal expansion coefficient. Candidate filler materials include inorganic salts such as calcium carbonate or calcium sulfate, pulverized metals such as aluminum or iron, ceramic such as silicon carbide or silicon nitride, short fibers such as glass or carbon, or woodmeal or resin powder.
U.S. Pat. No. 5,844,309 to Yukio et al. discloses an adhesive composition particularly useful in bonding a semiconductor device to a substrate. A filler material having a specific particle size distribution is incorporated into the resin component of the adhesive, thereby rendering the adhesive capable of completely filling hollows and gaps during thermal pressing of the semiconductor device to the substrate. The thermal conductivity of the adhesive may be enhanced by using fillers having excellent thermal conductivity, such as aluminum oxide, aluminum nitride, silicon nitride, silicon carbide, crystalline silica, fused silica and so forth. Where electrical conductivity is required, silver powder is used as the filler. Neither Yukio nor Kouichi discloses the low CME cyanate esters of the instant invention. Further, “excellent” is a relative term in that while the fillers may be much more thermally conductive than the adhesive, some on the list, fused silica for example, are poor thermal conductors compared to some of their ceramic peers, such as aluminum nitride.
Japanese Laid-open Patent Application No. JP 11-106,481 to Masahiro et al. and entitled “Underfill Material for Liquid Injection Sealing” also discloses an adhesive composition useful for semiconductor bonding. The composition includes a spherical inorganic filler having an average particle size of about 0.5-12 micrometers, with all particles smaller than 70 micrometers. The resin matrix includes an epoxy resin, a cyanate ester and a bisphenol compound.
Japanese Laid-open Patent Application No. JP 7-258,542 to Akio discloses a resin composition based upon a cyanate ester that is more resistant against separation of admixed microballoons during curing of the resin. Curing yields a homogeneous, lightweight material. The data sheet for what appears to be a related product refers to its composition as a syntactic foam. (BryteCor® EX-1541 Syntactic Foam, Bryte Technologies, Inc., Morgan Hill, Calif.). Typical applications of this syntactic foam include foam cores for space structures and net molded foam parts, e.g., for making tooling.
U.S. Pat. No. 4,931,496 to Qureshi et al. discloses a tough, damage tolerant fiber-reinforced composite based upon a cyanate ester resin formulation. The reinforcing fiber is a structural fiber such as glass.
U.S. Pat. No. 5,955,543 to Sachdev discloses an adhesive for bonding a die of an integrated circuit, the adhesive comprising an aryl cyanate ester resin and an additive that is a functionalized oligomeric/polymeric phenolic resin. The adhesive optionally may contain an electrically or thermally conductive filler. The electrically conductive filler is preferably a highly conductive metal such as silver, gold, copper or nickel, and preferably is in the form of flakes. The thermally conductive fillers can be AIN, SiO2, SiC, BN, the like, and mixtures thereof. The weight ratio of the resin/additive mixture to filler is preferably in the range of about 15:85 to about 50:50.
The prior art, as represented by the above-mentioned patent disclosures, suffers from a variety of shortcomings. As can be seen, many of the disclosures pertain to bonding integrated circuits (“dies”) to substrates, and many of these applications are for hermetic environments. Thus, expansion due to moisture absorption would not be as problematic compared to the product applications facing the instant inventors, where the adhesive joint will be exposed to moist environments, or even more challenging, where one side of the joint will be exposed to the moist environment. Thus, while some of these patents disclose low CTE fillers for a resin matrix, the matrix is not the low CME cyanate ester resin of the instant invention. Others disclose electrically or thermally conductive fillers for a cyanate ester resin system, but are silent on the issue of the resulting strength of the filled cyanate ester adhesive system. Perhaps in these patents, the strength of the joint is not critical because the component being bonded is relatively small and lightweight. In contrast, the instant inventors need a resin system for bonding large, heavy structures. Thus, the strength of the bond is important. At the same time, many of the above-mentioned properties are also desirable, such as high thermal conductivity. There are commercially available cyanate ester prepreg systems containing highly conductive structural reinforcement such as graphite fibers. There is also at least one commercially available cyanate ester based adhesive system containing thermally conductive boron nitride. Unfortunately, this composite adhesive system has a lap shear strength of only about 1000 psi (6.9 MPa) versus about 6000 psi (41 MPa) for the cyanate ester adhesive without such boron nitride filler.
In short, the prior art addresses some, but not all, of the issues confronting the instant inventors.
Thus, it is an object of the instant invention to produce a cyanate ester based adhesive featuring an additive filler that, upon curing, maintains at least a significant fraction of the tensile and shear strength of the neat cyanate ester.
It is an object of the instant invention to produce a high strength cyanate ester based composite adhesive that, upon curing, absorbs even less moisture than the neat cyanate ester.
It is an object of the instant invention to produce a high strength cyanate ester based composite adhesive that, upon curing, has a higher elastic modulus than the neat cyanate ester.
It is an object of the instant invention to produce a high strength cyanate ester based composite adhesive that, upon curing, has a higher thermal conductivity than the neat cyanate ester.
It is an object of the instant invention to produce a high strength cyanate ester based composite adhesive that, upon curing, has a lower CTE than the neat cyanate ester.